@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

tab .tab-close-button .toolbarbutton-icon,
tab .tabs-closebutton .toolbarbutton-icon
{
opacity: 0 !important;
}

tab:hover .tab-close-button .toolbarbutton-icon,
tab:hover .tabs-closebutton .toolbarbutton-icon
{
opacity: 1 !important;
}